An obscure district court lawsuit over the Affordable Care Act became a potent threat to one of the law's most popular provisions late Thursday, when the Justice Department filed a brief arguing that as of Jan. 1, 2019, the protections for people with preexisting conditions should be invalidated.

The Justice Department argued the judge should strike down the section of the law that protects people buying insurance from being denied insurance or charged higher premiums because of their health history.

The news rallied defenders of the law into action and raised questions among legal scholars about the likelihood it would succeed. Andy Slavitt, who led the Centers for Medicare and Medicaid Services under President Obama, called the action "the biggest health care news of the year" on Twitter.

BREAKING: The biggest health care news of the year.

The Trump DOJ tonight just told the courts to dismantle pre-existing conditions protections and other consumer protections.

According to a 2016 analysis by the Kaiser Family Foundation, approximately 52 million Americans under the age of 65 could find their access to health insurance at risk because of a wide range of preexisting conditions, from diabetes to cancer to pregnancy. Health insurers have for years been raising premiums, complaining about uncertainty and withdrawing from the business of selling individual insurance plans, and more changes could further destabilize the market.

"It would be essentially a return to what the individual market looked like before the ACA, where insurers would require applicants to fill out long questionnaires about their medical histories, and make decisions based on people’s health and how much to charge," said Larry Levitt, a senior vice president at the Kaiser Family Foundation. "Now we're in the situation where very sick people have gotten insurance, and so changing the rules means taking coverage away from people who genuinely need it."

America's Health Insurance Plans, a major lobby for insurers, said in a statement that although premiums for next year will increase because the individual mandate penalty for not carrying insurance is being zeroed out in 2019, the market has been stabilizing.

"Removing those provisions will result in renewed uncertainty in the individual market, create a patchwork of requirements in the states, cause rates to go even higher for older Americans and sicker patients, and make it challenging to introduce products and rates for 2019," the statement said.

Margaret A. Murray, the chief executive of the Association for Community Affiliated Plans, which represents plans for low-income and vulnerable populations, said that anyone who has bought individual insurance "and has had so much as a case of asthma in their past should be deeply unsettled by the choices this administration has made."

Major patient groups representing people with cancer, heart disease, diabetes, lung disease and multiple sclerosis, said in a joint statement that "striking down these provisions would be catastrophic and have dire consequences for many patients with serious illnesses.”

The court filing is a request to the judge, and will set off a long legal fight. It will add fuel to Democrats' efforts to make healthcare a campaign issue in the mid-term elections. The guarantee that people should be able to buy insurance regardless of their health history has been a popular provision of the divisive law -- one that President Trump has praised, calling the law's prohibition on denying insurance to sick people "one of the strongest assets" of the ACA in a "60 Minutes" interview before he took office.

In a June 2017 poll, Kaiser found that protection for people with preexisting conditions was an issue with bipartisan support.

Carolyn Y. JohnsonCarolyn Johnson is a reporter covering the business of health. She has written about the health care industry and the affordability of health care to consumers since 2015. She previously wrote about science at the Boston Globe. Follow